As the frequencies of modern computers continue to increase, the need to rapidly transmit data between chip interfaces also increases. To accurately receive data, a clock signal is often sent to help recover the data. The clock signal determines when the data should be sampled or latched by a receiver circuit.
The clock signal may transition at the beginning of the time the data is valid. The receiver circuit, however, may require that the clock signal transition during the middle of the time the data is valid. Also, the transmission of the clock signal may degrade as it travels from its transmission point. In both circumstances, a delay locked loop, or xe2x80x9cDLL,xe2x80x9d can regenerate a copy of the clock signal at a fixed phase shift with respect to the original clock signal.
FIG. 1 shows a section of a typical computer system component (100). Data (14) that is K bits wide is transmitted from circuit A (12) to circuit B (34) (also referred to as the xe2x80x9creceiver circuitxe2x80x9d). To aid in the recovery of the transmitted data, a clock signal (16) is also transmitted with the data (14). The circuits could also have a path to transmit data from circuit B (34) to circuit A (12) along with an additional clock (not shown). The clock signal (16) may transition from one state to another at the beginning of the data transmission. Circuit B (34) requires a clock signal temporally located some time after the beginning of the valid data. Furthermore, the clock signal (16) may have degraded during transmission. The DLL has the ability to regenerate the clock signal (16) to a valid state and to create a phase shifted version of the clock signal (16) to be used by other circuits. For example, the receiver circuit (34) may use the phase shifted version of the clock signal (16) as the receiver circuit""s sampling signal. The receiver circuit""s sampling signal determines when the input to the receiver circuit should be sampled. The performance of a DLL is critical, and the DLL must maintain a proper reference of time on the CPU, or generically, an integrated circuit.
FIG. 2 shows a block diagram of a typical DLL (200). Clock signal (201) is input to the DLL (200) to create a phased (i.e., delayed) output. Clock signal (201) is input to a voltage-controlled delay line (210) and to a phase detector (202). The phase detector (202) measures whether a phase difference between the clock signal (201) and an output signal, clk_out (217), of the voltage-controlled delay line (210) has the desired amount of delay. The phase detector (202) produces signals that control a charge pump (204). The phase detector (202) controls the charge pump (204) to increase or decrease its output current using up and down signals, U (203) and D (205). To ensure that the charge pump (204) maintains some nominal current output, the charge pump (204) is internally biased. The internal biasing of the charge pump (204) is dependent on bias signals, VBP (209) and VBN (211), generated from a bias generator (208) (discussed below). The up and down signals (203, 205) adjust the current output of the charge pump (204) with respect to the nominal current set by the bias signals (209, 211).
The charge pump (204) adds or removes charge from a capacitor C1 (206), which in turn, changes a voltage potential at the input of the bias-generator (208). The capacitor (206) is connected between a power supply, VDD, and a control signal, VCTRL (207). The bias-generator (208) produces the bias signals (209, 211) in response to the control signal (207), which, in turn, controls the delay of the voltage-controlled delay line (210) and maintains a nominal current output from the charge pump (204).
In FIG. 2, the voltage-controlled delay line (210) may be implemented using current starved elements. This means that the delays are controlled by modifying the amount of current available for charging and discharging capacitances. The linearity of a voltage controlled delay line""s characteristics determines the stable range of frequencies over which the DLL (200) can operate. The output signal (217) of the voltage-controlled delay line (210) represents a phase delayed copy of clock signal (201) that is then used by other circuits.
Still referring to FIG. 2, the negative feedback created by the output signal (217) in the DLL (200) adjusts the delay through the voltage-controlled delay line (210). The phase detector (202) integrates the phase error that results between the clock signal (201) and the output signal (217). The voltage-controlled delay line (210) delays the output signal (217) by a fixed amount of time such that a desired delay between the clock signal (201) and the output signal (217) is maintained.
Accordingly, proper operation of the receiver circuit (34 in FIG. 1) depends on the DLL (200) maintaining a constant phase delay between the clock signal (201) and the output signal (217).
According to one aspect of the present invention, an integrated circuit including a clock path arranged to carry a clock signal; a power supply path arranged to receive power from a power supply; a delay locked loop operatively connected to the power supply path and the clock path, comprising a capacitor, responsive to a phase difference between the clock signal and a delayed clock signal, for storing a voltage potential; a leakage current offset circuit operatively connected to the capacitor where the leakage current offset circuit is arranged to adjust the voltage potential; a programmable current source connected to the leakage current offset circuit where the programmable current source comprises a first current source and a first switch arranged to control the leakage current offset circuit; and a combinational logic circuit operatively connected to the programmable current source where the combinational logic circuit is arranged to selectively adjust the programmable current source.
According to one aspect of the present invention, a method for post-fabrication treatment of a delay locked loop including generating a delayed clock signal; comparing the delayed clock signal to an input clock signal; generating a current dependent on the comparing; storing a voltage potential on a capacitor dependent on the current; adjusting a leakage current of the capacitor using a leakage current offset circuit responsive to a programmable current source where the programmable current source comprises a first current source and a first switch arranged to control the leakage current offset circuit; and selectively adjusting the programmable current source using a combinational logic circuit operatively connected to the programmable current source.
According to one aspect of the present invention, an integrated circuit including means for generating a delayed clock signal; means for comparing the delayed clock signal to an input clock signal; means for generating a current dependent on the means for comparing; means for storing a charge; means for storing a voltage potential on the means for storing the charge dependent on the means for generating the current; means for adjusting a leakage current of the means for storing the charge; and means for selectively adjusting the means for adjusting.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.